A compressor is a machine that pumps gas in a compression chamber formed by a casing, and there are a screw compressor that pumps gas by the rotational motion of a rotor, a reciprocating compressor that pumps gas by the reciprocating motion of a piston, a scroll compressor that pumps gas by the turning motion of a spiral tooth-shaped member, and the like as the type of the compressor. A screw compressor will be described below as an example.
The screw compressor has a structure in which a pair of a male rotor and a female rotor rotate while meshing with each other in a compression chamber formed by a suction-side casing and a discharge-side casing and reduce the volume of a space formed between both the rotors and the volume of a space formed by the casing and the rotors while moving in an axial direction, thereby compressing fluid present in the spaces.
Moreover, as the screw compressor, there are an oil-cooled screw compressor in which oil is supplied into a casing as fluid and an oil-free screw compressor in which oil is not supplied.
The oil-cooled screw compressor is adapted so that a male rotor and a female rotor rotate while coming into contact with each other through an oil film. The oil-cooled screw compressor can prevent seizure between the rotors by reducing frictional heat, which is generated by the rotation of the rotors, with oil. Since oil mist is mixed in compressed air in the oil-cooled screw compressor, the oil-cooled screw compressor is not suitable for fields that require clean air, such as a food industry and an industry related with a semiconductor.
Meanwhile, since oil is not supplied at all in the oil-free screw compressor, the oil-free screw compressor can provide clean air but does not include seal using oil. Accordingly, the rotors rotate while both the rotors do not come into contact with each other and the rotors do not come into contact with the casing so that seizure is not caused between the rotors. For this purpose, timing gears are mounted on end portions of shafts of the rotors in order to apply torque to the rotors in the oil-free screw compressor. Accordingly, the structure of the oil-free screw compressor is more complicated than the structure of the oil-cooled screw compressor.
Further, since the rotors do not come into contact with each other in the oil-free screw compressor, there is a possibility that compressed air may flow backward to the suction side from a gap between both the rotors, a gap between the rotor and the casing, or the like and may adversely affect the performance of the screw compressor. For this reason, for the improvement of performance such as volumetric efficiency, in the oil-free screw compressor, a very small gap needs to be formed between both the rotors and between the rotors and the casing while both the rotors do not come into contact with each other and the rotors and the casing do not come into contact with each other. However, since the compressor actually has thermal expansion, machining errors, and the like, the non-contact between the rotors and the non-contact between the rotors and the casing cannot be completely achieved. Accordingly, a solid lubrication film is formed on the surface of the rotor as described in JP 5416072 B1 (Patent Document 1).
In addition, a factor affecting the reliability of the oil-free screw compressor is the generation of rust. Since the material of the rotor is stainless steel and a solid lubrication film is formed on the rotor, the generation of rust on the rotor hardly occurs as it is. However, since the material of the casing is cast iron, rust is easily generated on the casing.
As a treatment to be applied to the casing, there is an example for forming an abrasion-resistant coating disclosed in JP 2004-502095 W (Patent Document 2). An object of Patent Document 2 is to apply an abrasion-resistant coating to the rotors or the casing or both the rotors and the casing to suppress the leakage of compressed air. An example in which a nitride coating is used as the coating is disclosed.
Further, JP 2005-83235 A (Patent Document 3) discloses a scroll compressor of which the sliding surface is coated with metal nitride. An object of Patent Document 3 is also to ensure the abrasion resistance of a sliding portion and to improve sealability.